Lionfish such as Pterois volitans and Pterois miles are reef-dwelling predatory fish native to the Indo-Pacific region. In their native waters, lionfish have co-evolved with other fish such that their population is relatively stable.
Unfortunately, lionfish have been recently introduced into the Caribbean Sea and western Atlantic Ocean. With plenty of naïve prey and few predators able to kill them, the lionfish population has exploded and their range is expanding quickly. They eat small crustaceans and fish, notably including a critically endangered wrasse and immature snapper and grouper, two economically important Caribbean fish.
In response, fishermen in the Caribbean and along the southeast coast of the United States have begun catching lionfish to sell as human food. Also, divers often simply kill lionfish for sport and to help protect the lionfish prey species. Although a few young lionfish are netted to sell as aquarium specimens, mature lionfish are generally killed with a spear gun because their venomous spines can inflict painful and debilitating injuries on humans.
Lionfish show little fear of divers and can be approached to within a few feet, making them relatively easy to spear. Two obvious challenges related to spearing lionfish for food are getting the fish off the spear safely and minimizing damage to the reef. If a spear passes through a fish or misses it entirely, it is likely to hit the living coral where lionfish lurk. Because much of the motivation of spearing the dangerous-to-handle lionfish at all is to counteract the damage they do to the environment, it is undesirable to hunt them in a way that damages coral.
Two other practical concerns are 1) killing enough lionfish to make a difference in their population and 2) many subsistence fishermen in the Caribbean area cannot afford complex and expensive equipment.
To kill lionfish quickly, they are preferably killed with a single shot from a spear gun and removed quickly from the spear head. Conventionally, fishermen “bag” a lionfish by scraping it on the lip of a rigid container. Actual bags are no longer used much, because many people have been stung by spines protruding through the mesh or when removing the fish from the spear with an implement such as pliers.
Spear guns are typically powered by either a stretchy sling, such as rubber tubing or strips cut from an inner tube, or compressed gas, typically carbon dioxide from a canister attached to the spear gun.
Slings have the advantage of being useful for weeks until the rubber strips or tubing must be replaced, however, they have a slow firing rate because the elastic must be pulled onto a cocking mechanism by hand before each shot. Because a more powerful sling is proportionally harder to cock, a sling that reliably kills a lionfish with one shot from a safe distance is apt to be especially slow to prepare for firing.
Compressed gas, such as carbon dioxide, could provide sufficient pressure and volume for one-shot killing of a lionfish, but the volume of the cylinders is limited. Conventionally, a small “soda siphon” cylinder is attached underneath the forward portion of the spear gun or a larger canister is attached near the back end of the spear gun. The canister must be small enough that it does not make the spear gun difficult to swim with or to aim. Because the only way to determine how much gas remains in one of these canisters is to weigh it, a diver generally must carry spares. When a cylinder is exhausted on a dive, the canister may be discarded on the spot, likely to corrode and disperse metal compounds that are toxic to marine life. The ongoing expense of carbon dioxide canisters is too great for many fishermen.
In demonstrations of killing lionfish with spear guns powered by compressed carbon dioxide, it has been shown that multiple shots are usually required to kill or at least immobilize a lionfish to the point that it can be safely removed from the spear. This is inhumane, as well as slow, and wasteful of compressed gas.
A potential power source for a spear gun is the compressed breathing air used by SCUBA divers and normally carried on the user's back. This power source is not being used, though, because it is believed that using breathing air would cut the diver's time underwater by an unacceptable amount and that routing high pressure air through a spear gun could quickly exhaust the entire tank if the spear gun blew a leak. This would be especially dangerous if it happened at a depth greater than 30 feet.
There is a need for a weapon that can kill lionfish quickly and humanely, without undue physical exertion on the part of the fisherman or sport diver. Such a weapon must allow the fisherman to bag the killed fish without risk of stings, while preserving the fish in a condition suitable for sale to a restaurant.
There is further a need for a lionfish weapon that is inexpensive to buy and operate; such a weapon also preferably is incapable of harming a person, whether it accidentally strikes a person or is mistakenly discharged toward a person.
There is further a need for a lionfish weapon that does not harm coral or other living creatures, whether by impact or by generation of chemical or other waste.